1. Field of the Invention
The invention relates to single-ended electric lamps having an alkali-halide containing light source which produces ultraviolet radiation and is supported within an outer envelope by metallic support structure. More particularly, the invention relates to improvements for reducing photoelectron emission from the metallic support structure caused by ultraviolet radiation from the light source.
2. Description of the Prior Art
Photoelectron emission can be very detrimental in electric discharge lamps having an arcttube which contains an ionized plasma of alkali-halides during lamp operation, such as metal halide discharge lamps. The discharge vessel, or arc tube, of metal halide lamps is typically fused quartz glass and contains a filling comprised of mercury, sodium halide and other metal halides which are effective to contribute to the spectrum of light developed during lamp operation. A well-known characteristic of metal halide discharge lamps is the increase in lamp voltage that occurs over the lifetime of such lamps. Sodium ions diffuse through heated fused quartz glass, so that the sodium content within the lamp discharge vessel is progressively depleted during the course of lamp operation. The progressive loss of sodium results in a progressive increase in lamp operating voltage and also causes an unacceptably large increase in correlated color temperature (CCT) over the life of the lamp. The increase in correlated color temperature is particularly problematic in low wattage metal halide lamps, e.g. lamps having a rated wattage of 100W or less. At some time during the life of the lamp, its operating voltage may rise to a level greater than that provided by the lamp ballast, thus causing the lamp to extinguish. Sodium loss, and not deterioration of the lamp components, is frequently the determinant of lamp life.
Sodium diffusion through the arc tube is accelerated by any negative space charge within the outer envelope of the lamp. The negative space charge occurs if ultraviolet radiation from the discharge strikes metal components within the lamp and causes the production of photoelectrons.
Single-ended discharge lamps, i.e. lamps having an outer envelope with a lamp cap at only one end, employ metal frames for supporting the arc tube, typically axially, within the outer envelope and electrically connecting the lead-throughs at each end of the arc tube to respective terminals on the lamp cap. A principal frame component is an elongate metal support rod, or wire, extending within the lamp outer envelope past the arc tube and connected to the lead-through remote from the lamp stem. This support rod, along with other metallic frame structure, is exposed to ultraviolet radiation from the arc tube and emits a substantial flux of photoelectrons, especially in low wattage metal halide lamps where the support rod and other support structure is close to the arc tube because of the compact outer bulb employed. Accumulation of the photoelectrons causes the negative space charge and the attendant acceleration of sodium loss.
An established technique for reducing photoelectron production, and thereby reducing the rate of sodium loss, is to physically cover, wherever practical, metal components within the lamp outer envelope with material impervious to ultraviolet radiation and having a high photoelectric work function. U.S. Pat. No. 3,484,637 (van Boort et al) discloses a mercury vapor discharge lamp in which the metal support rod is covered by a refractory dielectric tube comprised of a ceramic of alumina and silica. The ceramic tube shields the covered portion of the metal rod from ultraviolet radiation, thereby reducing photoelectron production. A similar approach is disclosed in U.S. Pat. No. 3,780,331 (Knochel et al) in which a ceramic or fused quartz glass tube physically covers the support rod. Knochel further teaches the addition of a photoelectron collector and the use of a stainless steel rod having a chrome oxide surface, in place of the nickel plated iron support normally used. U.S. Pat. No. 4,171,498 (Fromm et al) likewise teaches the use of a fused quartz tube covering the support rod for reducing photoelectron emission. A fused quartz tube does not block the ultraviolet radiation from the conductor but is effective for trapping photoelectrons within the tube and substantially preventing photoelectrons from collecting on the arc tube.
In the above lamps having a covered support rod, the major part of the rod is straight and the ceramic or quartz glass tube covering the rod is straight. Major portions of the support structure remain uncovered and exposed to ultraviolet radiation because of sharp bends which cannot be covered with a single ceramic or glass tube, and/or short lengths which are impractical for cost/assembly reasons to provide with a tubular cover. These exposed portions include the bent end portion of the metal rod which extends from the stem press, the opposite end portion which is often connected to a dimple at the dome-end of the bulb, the metal conductor extending from the arc-tube lead-through to the metal support rod near the end remote from the lamp base, and the two lead-throughs extending from the arc tube.
Another alternative, disclosed in U.S. Pat. No. 4,866,328 (Ramaiah et al), is to cover parts of the metal support structure with a layer of zirconium oxide having a high photoelectric work function to reduce photoelectron emission. The zirconium oxide is granular and is applied mixed with an organic binder for adhering the zirconium to the metal support structure. However, to achieve acceptable adherence, the metal support structure needs to be sandblasted prior to coating and the coating must be baked to dry the binder, thus increasing the cost of the lamp.
Another approach to reducing photoelectron emission is to reduce the amount of metal in close proximity and in direct view of the arc tube. U.S. Pat. No. 3,424,935 (Gungle et al) discloses a single-ended metal halide lamp which eliminates the elongate support rod adjacent the arc tube by providing metallic structure only at the opposing ends of the outer envelope for supporting respective ends of the arc tube. The pinch seals of the arc tube are connected to the metal structure by conventional metal straps. A fine tungsten field wire extending proximate the curved envelope wall provides a conductive path between the lamp base and the far end of the discharge tube. Despite the elimination of the conductive support rod in Gungle, a substantial amount of photoelectrons are produced because the support structure at the ends is still exposed to ultraviolet radiation from the arc tube.
U.S. Pat. Nos. 3,662,203 (Kuhl et al) and 4,479,071 (T'Jampens et al) disclose double-ended metal halide discharge lamps in which the arc tube is enclosed in a narrow tubular outer envelope having a lamp cap at each end. The outer envelope has an inner diameter smaller than about three times that of the outer diameter of the arc tube. In the Kuhl patent, metallic holders are fixed to the arc-tube lead-throughs and have a plurality of fingers contacting the outer envelope to support and center the arc tube therein. Flexible current conductors connected to the holders extend through the outer envelope for energizing the arc tube. T'Jampens replaces the metallic holders of the Kuhl lamp with holders comprising boron nitride, which are impervious to UV radiation, thus eliminating a major source of photoelectrons.
For single-ended lamps having an elongate support rod, the most common commercial design remains the use of a ceramic or fused quartz tube over the straight portion of the support rod, with the attendant disadvantages previously discussed. Good design practice in reducing the amount of metal within the outer envelope may help reduce photoelectron production, but any practical arc tube support will necessarily include several metal parts of substantial mass and dimensions that are large relative to the overall lamp dimensions.
Accordingly, it is an object of the invention, in an electric lamp having a light source which produces ultraviolet radiation and an elongate support rod extending past the light source, to provide a practical and cost-effective means for more completely shielding the metal structure within the lamp envelope to suppress the emission of photoelectrons.